The present invention relates to electrodes for high-voltage electron beam tubes and, more particularly, to cathode cups for such tubes.
In high-voltage electron beam tubes such as, for example, x-ray tubes, an electron emitter or cathode is generally caused to emit electrons by a combination of relatively high voltage and heat. Control of emission of electrons may be by use of a "grid" whose potential with respect to the electron emitter may be varied to either accelerate or retard an electron beam. In an x-ray tube, the cathode may comprise a filament which is heated by electrical current to emit electrons and a surrounding cathode cup which acts as a grid to control electrons emitted by the filament. The cathode cup generally has one or more slots within which the filament, in the shape of a helical coil of wire, is positioned. Power is applied to the filament in order to force a heating current for generating electron emission.
The electrons emitted by the filament are controlled by voltage applied to the cathode cup. The potential between the filament and cathode cup may be several thousand volts. Accordingly, any contact between the filament and cathode cup may result in an electrical arc and welding of the filament to the cup at the point of contact thereby short-circuiting the filament and cup and resulting in uncontrolled electron emission. Unfortunately, such short-circuiting occurs relatively frequently as the filament is prone to move due to vibration created either mechanically by tube motion or electrically by high voltage breakdowns between the filament and cathode cup.
It is an object of the present invention to provide an improved cathode cup for an electron beam tube which reduces the risk of filament to cathode welding.
It is another object of the invention to provide an improved cathode cup for an electron beam tube which cathode cup is formed of a non-weldable material.